Intracardiac blood pressure sensing for research, diagnostic and treatment dates back to the early part of the 20th century, where early investigations utilized a canula or needle-based system with a mercury manometer. Using these techniques, pressure fluctuations in all 4 chambers of the heart have been successfully monitored. Critical diagnostic measurements of right heart systolic (pumping) and diastolic (resting) pressures can indicate disease conditions such as mitrial valve stenosis (stiffening), pulmonary artery hypertension, right heart weakness following myocardial infarction (heart attack), peripheral venus return failure (reduced preload) and electrical anomalies (arrhythmia or conduction).
Blood pressure can be also monitored through a fluid-filled tube or catheter where a diaphragm in the tip of the catheter deflects to transfer pressure to a pressure sensor external to the body. This method is typically used in either canula-based or catheter-based pressure sensors. These sensors are typically Piezo Resistive Technology (PRT) sensors. In contrast, sensors based on a Wheatstone bridge topology require high power levels and are typically too large for implantation. In addition, the sensors typically need to send the signals back to a remote device to capture the measured signals, often subject to signal degradation in the transmission process.
Another blood pressure sensing technology is the fiber optic blood pressure sensor. The sensor works through a small cavity embedded in the sensor tip, where the blood pressure is measured by observing the changes in length of the cavity using a measurement based on white light interferometry. Sensing light is transmitted to and reflected back from the detecting diaphragm and cavity of the sensor tip via a multimode fiber.
Testing information has been published for capacitor diaphragm-based pressure sensors, coupled to pacemakers, where the sensors are an integral part of pacemaker leads. The sensors are typically implanted to monitor intracardiac right heart pressure and have demonstrated a high correlation to standard balloon catheter measurements. These devices use a capacitive-based sensor in a catheter or pacemaker lead having a titanium deflectable sensing diaphragm at the tip. The diaphragm acts as one plate of a sensing capacitor and inside the diaphragm is an air-filled cavity with a second capacitor plate. The value of the capacitance is inversely proportional to the plate distance. As the pressure changes, the titanium diaphragm deflects, changing the plate spacing and therefore the capacitance. This change in capacitance can be detected by an electronic circuit.
Capacitive sensors are based on the equation:C=K·(A/D)where K is the dielectric constant, A is the capacitor plate area, and D is the distance between the 2 capacitor plates. With a metal diaphragm, the measurement of pressure is based on the plate deflection, or the change plate distance. Thus, the capacitance change per unit pressure is limited by the macroscopic motion of the plate. For high sensitivity, the plate movement must produce a significant capacitive change. This requires a thinner plate to allow the movement per unit pressure. However, reduced plate thickness complicates the diaphragm attachment method regardless of whether the diaphragm is welded, or adhesively bonded. Also, the capacitance of the wiring to separate electronics can be orders of magnitude greater than the diaphragm capacitance. This complicates the decoding electronics for pressure measurement. In addition, having the diaphragm directly contacting the sensing media (i.e., the liquid to be measured) can cause a shift of the capacitive value of the sensor from its initial nominal value. Thus, this design is susceptible to capative changes based on the sensing media with which it comes in contact.
Further, thermal effects, mechanical instability and aging effects contribute to an inaccuracy in the measurement taken by the capacitive-based sensor. For example, as the sensor ages, small movement in the wiring position or compression of the insulation may significantly alter the interconnect capacitance. This is seen as a change in the zero pressure reading or a drift of the reading with time. The range, accuracy and the repeatability of pressure measurement are not only limited by the motion of the diaphragm and the capacitance of the wiring, but also any thermally induced error. Since the diaphragm dimension can change by expansion and contraction due to thermal effects, accuracy is limited. The reproducibility of these thermal effects is also determined by the precision and reproducibility of the manufacturing process.
The current state of the art in intracardiac sensing is limited by the low level of signal output, remote sensing requirement, large physical size or custom fabrication for all designs. Most of the current state of the art sensors such as canula based, fluid filled catheters are not suitable for chronic (long term) unattended implantation. Others, such as the optical based sensors, require power levels too high for long term battery operation. Further, capacitor-based sensors require a secondary amplifier and detection circuit. These type of sensors may also be prone to long term drifting or lack of sensitivity.